1. Field of the Invention
The present invention relates to a recording apparatus applicable to a magnetic stylus recording system and various electrophotographic recording systems.
2. Description of the Related Art
Of various recording apparatuses, since an electrophotographic recording apparatus uses nonimpact recording, the apparatus produces low recording noise, can record clear characters, has a high recording speed, and has relatively low running cost. In recent years, therefore, the electrophotographic recording apparatus is used as an output terminal device of OA equipment, and its market has rapidly spread.
This electrophotographic recording apparatus will be briefly described below with reference to FIG. 1 which is a schematic view showing a recording section of a laser printer as one of electrophotographic recording apparatuses. As shown in FIG. 1, this electrophotographic recording apparatus uses a photoconductive drum 100. A charger 101 constituted by a corona charger uniformly charges the entire surface of the photoconductive drum 100 by, e.g., a negative charge to about -700 V. Laser light 102 is radiated on the photoconductive drum 100 in accordance with an image signal. Since the resistance of the photoconductive drum decreases only at a portion irradiated with the light, the negative charge at the portion irradiated with the laser light 102 is erased and an electrostatic latent image forms. Normally, a single semiconductor laser is used as a laser source, and light modulated in accordance with an image is scanned by a rotary polygon mirror. The electrostatic latent image thus formed is developed by a developing unit 103. That is, toner which is supplied from the developing unit 103 and consists of, e.g., coloring material fine particles negatively charged by reversal development is deposited on the portion of the electrostatic latent image formed on the photoconductive drum 100, from which the negative charge is removed, upon application of a development bias of about -500 V, thereby the electrostatic latent image is visualized. Recording paper 105 picked up by paper feed rollers 104 from a paper cassette (not shown) is fed in synchronism with the image signal and brought into contact with the photoconductive drum 100. In this contact portion, transfer of the visualized toner image onto the recording paper 105 is performed. A transfer charger 106 applies a positive charge from the lower surface side of the recording paper 105. This positive charge attracts the toner image formed on the photoconductive drum 100 by the negatively charged toner particles to the recording paper 105, thereby transferring the toner image onto the recording paper 105. The recording paper 105 to which the image is transferred is separated from the photoconductive drum 100 by a separating charger 107. A fixing unit 111 constituted by heat rollers 110 fixes the toner particles on the recording paper 105 by applying heat and a pressure to the toner particles, and in this manner recording is finished. Note that some toner particles are not transferred to the recording paper 105 but remain on the photoconductive drum 100. A cleaner constituted by a cleaning blade 108 scrapes off these residual toner particles, thereby cleaning the drum 100. Thereafter, an erasure lamp 109 constituted by, e.g., LEDs exposes the entire surface of the photoconductive drum 100 to erase the electric charge on the drum. In this manner, the electrophotographic recording apparatus forms an image through steps of charging, latent image formation, development, transfer, and fixing. The photoconductive drum is cleaned in the last cleaning step and resued. A general electrophotographic recording apparatus basically has the arrangement as described above although individual steps may be more or less modified depending on the type of apparatus.
A laser printer has been briefly explained above as a representative example of the electrophotographic recording apparatus. However, the electrophotographic recording apparatus is not limited to this laser printer, but various apparatuses using other light-emitting elements as a recording head for writing an electrostatic latent image have been developed and produced. A laser printer scans a pixel point with light generated by a single laser source by using a polygon mirror which mechanically rotates at a high speed or a hologram. In terms of the miniaturization and manufacturing cost of an apparatus, however, a solid scanning system using an array light source has attracted considerable attention recently. For example, an electrophotographic recording apparatus using a head in which light-emitting elements, such as LEDs, liquid crystal shutters, EL elements, plasma-emitting elements, or phosphors, or light-shutting elements are arrayed has been developed and put into practical use. An "optical printer" is a general term for these electrophotographic recording apparatuses. Such an electrophotographic recording apparatus is used as a printer or an output apparatus of a digital copying machine.
A conventional analog copying machine is also an electrophotographic recording apparatus, in which an original is irradiated with light generated by, e.g., a fluorescent lamp, and light reflected by the original is guided to a photoconductor to form an electrostatic latent image, thereby copying the original. A recording system called an ion flow recording or ion deposition recording is also one of electrophotographic recording systems. In this ion flow or ion deposition recording system, a dielectric is used in place of a photoconductor, and ions are ejected from arrays of small holes to record an electrostatic latent image on the dielectric.
Since the electrophotographic recording apparatus has various advantages as described above, a large number of these apparatuses are used as output terminal devices of OA equipment recently. Also, various systems for the apparatus of this type have been developed and put into practical use, and this has rapidly spread the market of the apparatus.
In these electrophotographic recording apparatuses, as described above, recording is performed through steps of charging, latent image formation, development, transfer, and fixing. One advantage of the electrophotographic recording apparatus is that energy required to form an electrostatic latent image is very small. For example, a latent image of one pixel point can be formed by applying an optical energy of about 10.sup.-6 to 10.sup.-5 J/cm.sup.2 to a photoconductor. To form one pixel point on recording paper using a thermal transfer recording apparatus, on the other hand, a large recording energy of about 2 to 6 J/cm.sup.2 is required. In this respect alone, it seems that the electrophotographic recording apparatus is very efficient and its consumption power is very small compared to that of a thermal transfer recording apparatus. In an actual electrophotographic recording apparatus, however, the consumption power is normally about 1.5 KW in the case of an apparatus capable of recording 8 to 12 sheets of paper per minute, and is a minimum of about 500 to 600 W in a low-speed apparatus capable of recording 4 sheets per minute. These values are equivalent to or larger than the value of consumption power of a thermal transfer recording apparatus. Of the steps of the recording process of the electrophotographic recording apparatus, those from charging to transfer of a toner image onto plain paper are realized by a very small energy. However, the last step of fixing the toner image on the recording paper requires a large energy, and this increases the consumption power of the electrophotographic recording apparatus as a whole. This fixing energy is, for example, about several tens J/cm.sup.2, which is a value about ten times the recording energy of a thermal transfer recording apparatus.
Recently, most electrophotographic recording apparatuses incorporate a fixing unit using heat and a pressure generated by a heat roll. This fixing unit using a heat roll is safe because it is free from a danger of a fire, and its heat capacity is large enough to keep image quality stable. The fixing unit also has an advantage that its fixing power is very high compared to that obtained by pressure fixing. However, since the heat capacity of the heat roll is set large, it takes a long time to raise the temperature of the heat roll up to a temperature required for fixing. Therefore, the apparatus cannot be used immediately after its power switch is turned on but requires a warm-up time of about several minutes before it can be used. In addition, a heater which consumes large power is required because the heat capacity of the heat roll is large, so a lamp of about 500 to 1,000 W is generally incorporated in the roller. That is, since the conventional electrophotographic recording apparatus uses a heat roll having a large heat capacity as a fixing unit, the apparatus requires a large consumption power and a long warm-up time. Especially when miniaturization of the electrophotographic recording apparatus is taken into account, the apparatus has problems that a heat roll having a large consumption power and a large heat capacity is used as a fixing unit and that such a fixing unit must be provided independently of an image forming drum. In addition, to prevent an influence of heat on the image forming drum, the fixing unit and the image forming drum must be arranged as far as possible. These problems make it difficult to miniaturize the electrophotographic recording apparatus.
Several image forming methods using conductive magnetic toner and recording electrodes were also reported in the past. Examples of the method are described in A. R. Kotz: J. Appl. Phot. Eng. 7, (2), page 44, 1981 and K. Okuna et. al.: Proc. Japan Hardcopy 91, page 117, 1991. These recording methods are almost the same in the process of forming an image. FIG. 2 shows the recording principle of these methods. Referring to FIG. 2, an image carrier 100 is constituted by a recording layer 101 consisting of an insulating layer and a conductive layer 102 connected to the ground potential. Carrying of conductive magnetic toner 31 is performed by a fixed sleeve 33 and an internal magnet roller 32 of the sleeve 33. Since the toner is magnetic toner, it is held in a state forming a magnetic brush formed on the sleeve 33 by the magnetic force of the magnet roller 32. In this condition, the toner 31 is carried on the fixed sleeve 33 outside the magnet roller 32, which is magnetized to have N and S poles alternately, by rotation of the magnet roller 32 in a direction opposite to the direction of rotation of the magnet roller 32 (indicated by an arrow of a broken line in FIG. 2). Recording electrodes 34 are adhered to a position on the fixed sleeve 33 opposite to the image carrier 100. The recording electrodes 34 are arrays of a large number of electrodes, and each electrode 34 has a driver 35 for switching on/off a recording signal in accordance with image data. When a voltage is applied to the recording electrodes 34, an electric charge flows through the layer of the toner 31 because the toner 31 is conductive toner, and this charge reaches toner particles in contact with the recording layer 101 of the image carrier 100. The capacitance of this toner is charged. An electric charge having a polarity opposite to that of the recording voltage is induced in the conductive layer 102 of the image carrier 100. When a Coulomb force generated between the charge of the toner and the charge of the opposite polarity induced in the conductive layer 102 becomes larger than the magnetic force holding the magnetic toner 31, the toner particles are transferred onto the image carrier 100 to form an image 36.
This recording system is similar to an electrophotographic printer or an electrostatic recording apparatus in a sense that the image 36 is formed on the image carrier 100 by using the toner 31. In the electrophotographic printer or electrostatic recording apparatus, however, an electrostatic latent image is first formed on an image carrier and then developed using toner to form a visual image. In the method using conductive magnetic toner, on the other hand, a toner image is formed directly on an image carrier without forming any latent image. Therefore, since no discharge phenomenon is used in this recording apparatus unlike in the electrophotographic printer or electrostatic recording apparatus, the apparatus requires neither high-voltage parts, such as a charger, nor a high-voltage power source. The voltage to be applied to the recording electrodes is also a very low voltage of about 30 V. That is, this recording apparatus has a characteristic feature in that all the voltages used are low voltages. In addition, although a part like a charger produces a harmful substance such as ozone, this recording apparatus does not use a charger, so there is no possibility of producing such a harmful substance. Furthermore, since the number of recording steps is naturally decreased, the recording apparatus can be simplified and miniaturized. Also, this recording apparatus does not perform optical recording using a photoconductor unlike an electrophotographic printer. This makes it possible to use an inexpensive image carrier having a long service life and makes it unnecessary to provide a dark space. The nonuse of optical parts realizes miniaturization of the apparatus.
As described above, the method of forming a toner image directly on an image carrier by using conductive magnetic toner has a large number of advantages compared to a conventional electrophotographic recording apparatus or electrostatic recording apparatus. However, practical examples of this recording apparatus are very few compared to those of the electrophotographic recording apparatus or electrostatic recording apparatus. One of the largest reasons for this is that the use of conductive toner makes it very difficult to transfer a toner image formed on an image carrier onto plain paper. To transfer a toner image from an image carrier to plain paper, an electrostatic transfer method is generally adopted in an electrophotographic recording apparatus. In this method, plain recording paper is fed to a transfer unit and brought into tight contact with an image carrier in this unit. An electric field by which charged toner is electrostatically attracted to the recording paper is formed between the recording paper and the image carrier. The electrostatic transfer method is classified into corona transfer and roller transfer in accordance with electrostatic field forming means. In the corona transfer method, a corona charger applies an electric charge having a polarity opposite to that of charged toner to the lower surface of recording paper. The charged toner is transferred to the recording paper by an electrostatic force acting between the charged toner and the charge of the opposite polarity. In the roller transfer method, on the other hand, a conductive rubber roller applied with a voltage or a dielectric roller obtained by forming a dielectric film on the surface of a conductive rubber roller is urged against the lower surface of recording paper to form an electric field. The basic principle of electrostatic transfer is substantially the same in these two methods; charged toner particles are attracted and transferred from an image carrier to recording paper by an electrostatic force.
When, however, the electrostatic transfer as described above is performed using conductive toner, image quality is significantly degraded. The reason for this is that the electric charge of toner particles leaks to recording paper because the toner is conductive, and this makes it impossible to reliably hold the toner particles on the recording paper by an electrostatic force. This disables transfer of a toner image onto plain paper. Even if an image is transferred, the transferred image is a disturbed one.
To solve these problems, a transfer method other than the electrostatic transfer is adopted when conductive toner is to be used. Normally, a pressure transfer method is used. In the pressure transfer method, a high pressure is applied to a toner image and recording paper to bring them into tight contact with each other, and toner particles softened in this manner are transferred to the recording paper. Although the pressure transfer method has an advantage of a high transfer efficiency, a very high pressure must be applied in this method. This requires a high mechanical strength or a large drive torque. Therefore, it is difficult to use this pressure transfer method in a compact recording apparatus. The method also has a drawback in that only an image carrier having strength high enough to withstand a high pressure can be used. In addition to the pressure transfer method, it is also possible to use a method in which toner particles are transferred to an adhesive intermediate medium by using the adhesion of the medium, and the transferred toner image is further transferred from the intermediate medium to recording paper and fixed on it by using heat or the like. This method has advantages of a high transfer efficiency and little degradation in image quality. However, since the method requires retransferring a toner image twice, an apparatus for this method is increased in size and complicated. It is also possible to adopt a method in which a toner image is transferred not to plain paper but to special insulating recording paper in order to prevent a leakage of an electric charge of charged toner particles.
As described above, although the recording apparatus using conductive toner has several advantages, it has not been widely used because transfer to plain paper is not easy.